The present invention relates to very high solids slurries of TiO2, and processes for preparing same.
Titanium dioxide is produced commercially in two crystalline polymorphic forms, namely the rutile form which can be produced by both the chloride and sulfate processes and the anatase form which is produced by the sulfate process. The anatase and rutile forms of TiO2 differ not only in their crystallographic structures but also in the type and amount of surface and other impurities resulting from their differing methods of production. These differences are such that generally the rutile form produced by the chloride process and the anatase form produced by the sulphate process are processed and refined into finished pigments by significantly different methodologies, with one procedure often not being effective with both forms of TiO2. This invention is applicable to both the rutile and anatase forms of TiO2 produced by the sulphate process, but provides especially notable improvements when practiced with anatase TiO2.
Sulfate anatase TiO2 is generally produced by digesting a titaniferous ore, such as ilmenite ore, ore concentrates and blends thereof, in concentrated sulfuric acid to form an iron sulfate-titanium sulfate solution which is subsequently cooled to precipitate out the iron sulfate as copperas. The clarified and concentrated titanium sulfate solution is then heated to hydrolyze and precipitate out hydrous TiO2. The hydrous titanium dioxide is calcined at an elevated temperature sufficiently high, i.e. at least about 450 to about 1000xc2x0 C., to partially or completely dehydrate the titanium dioxide to form a water insoluble anatase, or rutile TiO2 pigment, as the case may be. The crude product resulting from the calcination contains a substantial amount of large particles and grit which must be broken up by either dry or wet grinding.
After dry or wet grinding of TiO2 pigment to a desired pigmentary particle size, the pigment may be shipped to end users as either a dry powder or an aqueous slurry. The shipment of high solids aqueous slurries has certain advantages to end-users because they do not have to undertake the rather complex and time consuming process of mixing titanium dioxide pigments and water for use in making paper, coatings, paint or other products. However, it also presents disadvantages in that the shipping of large amounts of water over large distances is an inefficient use of resources. Another disadvantage is that the water present in lower solids slurries may create excess dilution in certain applications, such as high solids paper coating, requiring costly and time consuming drying or slowing of paper coating equipment to remove the excess water.
It is known that crude TiO2 may be compacted and crushed or ground after its production to break-up oversized particles, grit, lumps and aggregates. High solids slurries of anatase TiO2 containing greater than 73% anatase TiO2 solids, however, have heretofore not been commercially available.
For instance, U.K. Patent No. 1,204,326 describes subjecting calcined TiO2 pigment discharge to a pressure rolling step and then a disintegration step which may be a wet milling step. Suspensions of 27 and 35.5% TiO2 solids and paint containing 37% TiO2 solids only are described in the examples.
U.S. Pat. No. 5,290,352 discloses the same process as U.K. Patent No. 1,204,326 for preparing inorganic pigment suspensions, including TiO2 pigment suspensions. Suspensions having solids contents of from 5 to 75% by weight are claimed. However, suspensions of 20 to 25% TiO2 solids only are described in the examples.
U.S. Pat. No. 5,154,362 discloses a method for crushing brittle material involving first crushing the material in a roller mill to form agglomerates which are then broken up, screened, and passed to a dry or wet mill for further grinding. In U.S. Pat. No. 5,520,340, compacting inorganic pigment on a drum compactor, then de-agglomerating the compacted pigment before it is jet milled is disclosed. In neither patent is the formation of pigment suspensions disclosed.
Resort to more involved processes to achieve high solids TiO2 slurries have been attempted. U.S. Pat. No. 4,280,849 discloses a process for preparing high solids titanium dioxide slurries in which a first pressure filter cake of a coated TiO2 is mixed with a dispersing agent, preferably in a sand mill, then dewatered under superatmospheric conditions to produce a second filter cake that is reslurried to form a high solids slurry of 60 to 75% coated titanium dioxide. Similarly, U.S. Pat. No. 4,978,396 discloses a method said to be useful in obtaining 70 to 80% solids slurries of rutile or anatase titanium dioxide in which an initial low solids slurry of wet milled and hydroclassified pigment is dewatered under superatmospheric conditions to produce a wet filter cake that is reslurried to form the high solids slurry. The lone example of this patent obtained a solids content of 71%.
U.S. Pat. No. 4,288,254 discloses a process for preparing a high solids slurry of rutile TiO2 by passing a slurry of wet milled or, dry micronized, dispersant-coated TiO2 through a high sheer mill to de-agglomerate the TiO2. The sole example shows the preparation of a slurry having at most 64.6% solids.
The present invention relates to high solids anatase TiO2 slurries comprising greater than about 75% by weight of anatase TiO2 pigment and water. The high solids slurries are free-flowing and pumpable.
The high solids result from dry milling anatase TiO2 prior to mixing with water to form the slurry.
Accordingly, the present invention also provides a process for producing a high solids anatase TiO2 slurry. The process comprises the steps of a) dry milling anatase TiO2 pigment and b) mixing the dry milled anatase TiO2 pigment with water thereby forming a slurry comprising greater than about 75% anatase TiO2 pigment. Though grinding mills, such as Raymond mills, and other dry mills are usefully employed in the present invention, particularly advantageous is the use of crushing mills, especially pressure rollers.
The slurries of the present invention are fluid and stable at a wide range of pH levels, which was heretofore unachievable with high solids anatase TiO2 slurries. Anatase TiO2 slurries comprising greater than about 74% solids and having a pH of less than about 9 are a particularly desirable achievement.
Therefore, the present invention provides a high solids slurry of anatase TiO2 pigment comprising greater than about 74% by weight of anatase TiO2 pigment and water; and having a pH of less than about 9.
Also, forming the high solids slurry in the presence of a preferred acrylic acid dispersant enhances the viscosity stability and reduces the tendency of the slurry to flocculate.
Thus, the present invention provides a high solids slurry of anatase TiO2 pigment additionally comprising a dispersant selected from the group consisting of i) acrylic acid homopolymers and ii) copolymers that are predominantly acrylic acid and contain at least one comonomer selected from the group consisting of maleic acid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid, acrylamide, acrylonitrile, ethylene, propylene, styrene and esters of the acids, wherein the homopolymers or copolymers have been partially or completely neutralized with a neutralizing agent having a monovalent group and at least one neutralizing agent having a polyvalent group.
The present invention also provides a process of preparing a high solids slurry of anatase titanium dioxide pigment which comprises greater than about 75% anatase TiO2 pigment, comprising a) dry milling anatase TiO2 pigment; and mixing said dry milled TiO2 pigment with water in the presence of a dispersant selected from the group consisting of i) acrylic acid homopolymers and ii) copolymers that are predominantly acrylic acid and contain at least one comonomer selected from the group consisting of maleic acid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid, acrylamide, acrylonitrile, ethylene, propylene, styrene and esters of the acids, wherein the homopolymers or copolymers have been partially or completely neutralized with a neutralizing agent having a monovalent group and at least one neutralizing agent having a polyvalent group, thereby forming said slurry.
Further, the process of the present invention is also effective in making high solids slurries of rutile TiO2 produced by the sulfate process, as well as high solid slurries of anatase TiO2.
Thus, the present invention further provides a process for preparing a high solids slurry of TiO2 produced by the sulphate process which slurry contains greater than about 75% TiO2 pigment, comprising dry milling the TiO2 pigment; and mixing the dry milled TiO2 pigment with water in the presence of an acrylic acid dispersant thereby forming the high solids slurry.
In accordance with the present invention high solids slurries of anatase TiO2 are provided with a solids content in excess of any heretofore accomplished. Typically, commercial anatase TiO2 slurries are produced and marketed at an anatase TiO2 solids content of no greater than 73% on a weight basis. This has been thought to be the maximum amount of anatase TiO2 that could be incorporated into a free-flowing slurry.
Slurries having an anatase TiO2 solids content of greater than about 75%, preferably greater than about 77%, and more preferably greater than about 80% by weight are readily provided by practice of the present invention. As most end uses of high solids anatase TiO2 slurries should not require slurries having in excess of about 85% anatase TiO2 solids, and as the time and energy expended in making slurries increases with the solids content, slurries containing up to about 85% anatase TiO2 solids on a weight basis are preferred, and those containing up to about 81% anatase TiO2 solids on a weight basis are more preferred. Of course, slurries of less than about 75% anatase TiO2 solids, for example slurries having anatase TiO2 solids contents of from about 60 to about 75%, also may be prepared in accordance with the present invention, if so desired.
The high solids slurries can be shipped to manufacturers for use in the making of products, such as paper, paints, and coatings, or for use in other pigmentary applications, or they may be first subjected to further processing such as wet milling, for example in a sandmill or a horizontal media mill, to provide a slurry with a more desirable mean pigmentary particle size or other attribute.
The high solids anatase TiO2 slurries of the present invention have been observed to be, when subjected to further processing, more easily processed than prior slurries. It also has been observed that the present slurries are more readily pumped and dispersed and show little or no dilatency problems.
These latter properties can be attributed to the free-flowing viscosity and rheological properties of the high solids anatase TiO2 slurries of the present invention. The slurries may have viscosities spanning a wide range depending on the use of which the slurry is to be made and the processing conditions and equipment to which it will be subjected (e.g. pumping, mixing, milling, filtering). High solids anatase TiO2 slurries of the present invention may have any viscosity desired, however, as anatase TiO2 slurries generally tend to become not readily pumpable at a Brookfield viscosity much greater than about 1500 cps., slurries having a Brookfield viscosity of less than about 1500 cps. are preferred, more preferred are slurries with a Brookfield viscosity of less than about 1000 cps. At very low viscosities, anatase TiO2 tends to unduly settle out of the slurries. Thus, slurries having Brookfield viscosities of between about 400 to about 800 cps. are further preferred. (In the present application all Brookfield viscosities are given as measured using a Model LVT Brookfield viscometer with a number 3 spindle revolving at 60 rpm.)
In addition, high solid anatase TiO2 slurries having a Hercules rheology such that their Hercules torque values are less than about 18xc3x97105 dynes-cm., at 4400 rpm., are preferred because such slurries are easily handled in mixers, plant process lines and pumps. More preferred are slurries having Hercules torque values from about 2 to about 12xc3x97105 dynes-cm. Slurries with higher Hercules torque values, especially slurries that are measured as being dilatent at less than about 1100 rpm., present handling difficulties in processing. (In the present application all Hercules torque values are given as determined using a Model ET-24-6 Hercules Hi-Shear Viscometer equipped with an xe2x80x9cAxe2x80x9d bob operating in accordance with TAPPI (Technical Association of the Pulp and Paper Industry) test method T648, except that the viscometer rpm was automatically increased from 0 to 4400 rpm during the measurement cycle. Torque values at 4400 rpm were read directly from the chart recorder. For slurries that were measured as being dilatent, the rpm at which the slurry was measured as being dilatent was recorded.)
The high solids contents, free-flowing viscosities and improved rheological properties of the present anatase TiO2 slurries enable many end-use improvements and expand the limits within which many end users, such as paper manufacturers, were heretofore forced to operate.
The high solids anatase TiO2 slurries of the present invention may include a dispersant to aid in their viscosity stability and resistance to flocculation. Organic and inorganic dispersants useful in the slurries include alkanolamines and phosphates. Representative alkanolamines include isopropanol amine, diisopropanol amine, 2-amino-2-methyl-1,3-propanediol, 2-amino-2-methyl-1-propanol, aminoethanolamine, and triethanolamine. Representative phosphate dispersants include tetrapotassinm pyrophosphate, tetrasodium pyrophosphate, sodium tripolyphosphate, and potassium tripolyphosphate. The dispersants may be present in amounts up to about 5%. Preferably the dispersants are present in amounts of from about 0.05 to about 2%.
The abovementioned dispersants, while being effective in the subject invention, however, have been found to suffer from several shortcomings, most notably, poorer than desirable calcium tolerance, viscosity stability, foaming tendencies and tendency to flocculate in their usage applications when compared to acrylic acid dispersants. Accordingly, in practice of the present invention, it is preferred that the slurries of anatase TiO2 are provided with improved slurry viscosity stability, calcium and magnesium tolerance, foaming tendency and tendency to flocculate by containing a sufficient quantity of an acrylic acid dispersing agent selected from the group consisting of acrylic acid homopolymers and copolymers that are predominantly acrylic acid and contain at least one comonomer selected from the group consisting of maleic acid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid, acrylamide, acrylonitrile, ethylene, propylene, styrene and esters of the above acids, which have been partially or completely neutralized with a neutralizing agent having a monovalent group and at least one neutralizing agent having a polyvalent group. Such preferred dispersants are more fully described in U.S. patent application Ser. No. 08/557,953, issued as U.S. Pat. No. 5,746,819, which disclosure in full is incorporated herein by reference.
The preferred acrylic acid dispersants useful in the practice of the present invention is produced by known processes of polymerization and has a molecular weight between 300 to 100,000, preferably between 1,000 to 20,000 and most preferably between 2,000 to 10,000.
The acid sites of the homopolymer or copolymer may be neutralized with combinations of neutralizing agents having monovalent or polyvalent groups. The neutralizing agent containing the monovalent group has a group selected from alkali metal cations, ammonium, aliphatic or cyclic primary, secondary or tertiary amines. The most preferred monovalent cations are sodium and potassium. The neutralizing agent containing the polyvalent group has a group selected from alkaline earth cations, zinc and aluminum. The most preferred polyvalent cation is calcium.
The degree of neutralization of the acid polymers by the neutralizing agents with monovalent cations is between 0.02 to 0.90, most preferably between 0.05 to 0.50. The degree of neutralization by the neutralization agents containing polyvalent cations is between 0.10 to 0.98, most preferably between 0.95 to 0.50.
The degree of neutralization can be achieved by neutralizing the acid polymer with a mixture of neutralizing agents containing monovalent and polyvalent cations or mixing the appropriate proportions of acid polymer neutralized with the monovalent cation containing neutralizing agents with the acid polymer neutralized with the polyvalent cation containing neutralizing agents.
The neutralized acid homopolymer or copolymer is contained in the slurry at an amount sufficient to achieve a fluid, stable viscosity slurry up to about 5% based on the weight of the anatase TiO2 present in the slurry. The preferred content of the neutralized acid homopolymer or copolymer is 0.05 to 1.0%, and the most preferred range is 0.10 to 0.60% based on the weight of the anatase TiO2 present in the slurry.
The high solids anatase TiO2 slurries of the present invention also may have pH values spanning a wide range from about 5 to about 12. Though the desired pH for any slurry naturally will depend on the use of which the slurry is to be made, high solids anatase slurries at low pH values are especially advantageous.
Today, commercial high solids anatase slurries, which have a maximum solids content of about 72.5%, have pHs in excess of about 9. At pHs at or below about 9, these slurries are too viscous to be useful. In contrast to such slurries, the slurries of the present invention, even at solids levels greater than 74%, and at pHs less than about 9, are free-flowing and pumpable.
Production of high solids anatase TiO2 slurries over a broad range of pH values enables a slurry to be tailored to the particular needs of its use, eliminating or greatly reducing the need for costly and time consuming neutralization or pH adjustment of the slurry or the products made from it.
Accordingly, particularly useful high solids anatase TiO2 slurries are those having an anatase TiO2 solids content of more than about 74%, preferably more than about 77%, and most preferably more than about 80%, and having a pH less than about 9. As the time and energy expended in making slurries increases with the solids content, and it is not expected that most end users will require anatase TiO2 slurries having a solids content in excess of 85%, slurries containing up to about 85% anatase TiO2 solids on a weight basis are preferred, and those containing up to about 81% anatase TiO2 are more preferred. Of course, low pH slurries having anatase TiO2 solids contents less than about 74%, for example from about 60 to about 74% solids, also may be prepared in accordance with the present invention, if so desired.
As many present day pigmentary applications for anatase TiO2 , such as paper coating and paper making, are carried out at pH levels of from about 7 to 9, high solids anatase TiO2 slurries having a pH of from about 7 to about 9 are preferred. Further, as acidic or only slightly basic high solids slurries having more than about 74% anatase TiO2 solids by weight have heretofore not been commercially available, such slurries having a pH of from about 5 to about 9 are preferred, and those having a pH of from about 5 to about 7 are even more preferred.
The low pH, high solids anatase TiO2 slurries of the present invention may include a dispersant to aid in their viscosity stability and resistance to flocculation. Any of the organic or inorganic dispersants abovementioned are useful, with the acrylic acid dispersants being preferred. Of the preferred dispersants, the more preferred are as herein above described.
A particularly useful and preferred high solids anatase TiO2 slurry in accordance with the present invention is a slurry containing at least about 74% by weight of anatase TiO2 and having a Brookfield viscosity of less than about 1500 cps, a Hercules torque value of less than about 18xc3x97105 dynes-cm., and a pH of less than about 9.
The high solids anatase TiO2 slurries of the present invention are prepared by first dry milling anatase TiO2 pigment and then mixing the dry milled pigment with water to form a slurry.
The anatase TiO2 pigment that is dry milled may be any dry pigment, whether it has or has not been previously dry ground, crushed, or milled. However, it is generally unnecessary to grind, crush, or mill anatase TiO2 pigment prior to it being dry milled in accordance with the present invention to achieve high solids anatase TiO2 slurries. Therefore, it is preferred that the anatase TiO2 pigment be crude discharge product from the calciner, where it was produced, which has not been subjected to any prior intervening processing step that substantially grinds, crushes, or mills the discharge product. Further, the anatase TiO2 pigment preferably is not slurried or wet milled prior to being dry milled.
The anatase TiO2 pigment may be dry milled using any number of dry milling devices familiar to those skilled in the art of pigmentary slurry production, including grinding mills, such as Raymond mills, ball mills, attrition mills, and impact mills, and crushing mills, such as pressure rollers. Grinding mills, in particular Raymond mills, are particularly useful in dry milling anatase TiO2 pigment. However, the more preferred dry milling devices are crushing mills, especially pressure rollers.
Pressure rollers are relatively simple process equipment used for crushing and compacting (agglomerating) solid materials to produce pellets, tablets and briquettes in agglomeration operations. Commercial pressure rollers typically consist of two heavy rollers turning on parallel horizontal axes. TiO2 is fed between the rollers where it is caught and broken by compression then dropped out below. The rollers turn toward each other at approximately the same speed and have relatively narrow faces and large diameters so that they can xe2x80x9cnipxe2x80x9d the TiO2 lumps. Typical rollers have faces from about 1.5 to about 18 inches and diameters from about 8 to about 36 inches. To allow unbreakable material to pass without damaging the equipment, generally at least one roller is spring mounted. Pressure rollers can produce large quantities of crushed TiO2 at relatively low cost.
Pressure rollers suitable for carrying out the present invention include those manufactured by The Fitzpatrick Company (832 Industrial Drive, Elmhurst, Ill., 60126), K. R. Komarek, Inc.(1825 Estes Avenue, Elk Grove Village, Ill. 60007), and Hosokawa Bepex Corporation (333 N. E. Taft Street, Minneapolis, Minn. 55413).
In pressure rolling, anatase TiO2 pigment is fed into the nip or gap between two rolls rotating at approximately equal speeds. The anatase TiO2 is pressed at a high pressure sufficient to form agglomerates of the TiO2 by which it is simultaneously crushed. As the roller pressure increases, the mean particle size decreases. Suitable pressures are from about 750 to about 30,000 lb/in., preferably between about 1,000 and about 20,000 lb/in., and more preferably between about 4,000 and about 15,000 lb/in. Pressure rollers using grooved or rough surface rollers are preferred as they are more efficient at reducing the particle size of anatase TiO2 than pressure rollers using smooth rollers. Further, it is preferred that the anatase TiO2 pigment be passed through the pressure rollers only once.
Following the dry milling, the milled anatase TiO2 is mixed with water to form the slurry. Typically the milled anatase TiO2 pigment is mixed with water in a batch mixer and stirred until the desired solids content, degree of dispersion and viscosity is achieved. Preferably, the milled anatase pigment is added with intense mixing, using any of a number of mixing devices familiar to those skilled in the art of pigment slurry production. An agitator with a Cowles type blade or turbine impellers on a mixer of suitable horsepower are typical arrangements.
Tap water or deionized water can be used for the process of the invention. The pH of the water (or of the mixture of any dispersant or additive in the water) can vary, for example, from about 7 to about 12, preferably in the range of 8.5 to 11, and most preferably in the range of 9 to 10.
Anatase TiO2 pigment may be mixed with the water in the presence of a dispersant to aid in the forming of the high solids slurry of anatase TiO2 pigment. Suitable dispersing agents include any of the organic or inorganic dispersants abovementioned, with the acrylic acid dispersants being preferred. Of the preferred dispersants, the more preferred are as herein above described.
The preferred dispersants, selected from the group consisting of i) acrylic acid homopolymers and ii) copolymers that are predominantly acrylic acid and contain at least one comonomer selected from the group consisting of maleic acid, methacrylic acid, itaconic acid, crotonic acid, fumaric acid, acrylamide, acrylonitrile, ethylene, propylene, styrene and esters of the acids, wherein the homopolymers or copolymers have been partially or completely neutralized with a neutralizing agent having a monovalent group and at least one neutralizing agent having a polyvalent group, may be used in amounts up to about 5% based on the weight of the anatase TiO2 pigment to be slurried. Preferably the dispersants are used in amounts of from about 0.05 to about 1.0% by weight of the anatase TiO2.
If one of the preferred acrylic acid dispersants is employed in the process for producing the high solids anatase TiO2 slurry, the dispersant is added to the water first, the pH of the water mixture is then preferably adjusted to a value greater than 7, and then the anatase TiO2 is added. Other dispersants may be added before, after, or with the addition of the anatase TiO2 pigment.
Another option is that before mixing the dry milled anatase TiO2 pigment with water, it may be de-agglomerated to break-up any agglomerates and yield a finely divided powder. De-agglomerating imparts better dispersability to the anatase TiO2 pigment thereby aiding in the formation of the slurry. As agglomerates are dense and tend to settle out of a slurry, de-agglomerating also aids in preventing the clogging of pipes, filters, pumps, mills, etc., should the high solids anatase TiO2 pigment slurry be further processed. In particular, de-agglomerating helps to prevent agglomerates of the anatase TiO2 from setting in, for example, unmixed storage tanks.
De-agglomerating may be accomplished by using any number of devices and methods known to those skilled in the art of pigment slurry production. Pulverizing the dry milled anatase TiO2 pigment using a Fitz mill (available from The Fitzpatrick Company) is particularly suitable for yielding a finely divided powder.
The high solids anatase TiO2 slurries may be optionally wet milled to a desired mean pigmentary particle size. The resultant slurry is a high solids anatase TiO2 pigment slurry within the purview of the present invention.
Wet milling of high solids anatase TiO2 slurries may be performed using any of a number of wet milling devices and techniques known to those skilled in the art of making pigmentary slurries. The wet milling may be carried out in the presence of a grinding medium or in the absence of a grinding medium. Suitable wet mills include vertical sandmills, horizontal media mills, fluid energy mills, and high speed dispersers. Preferred are sand mills and horizontal media mills.
The process of the present invention also is applicable in the formation of high solids slurries of rutile titanium dioxide pigment produced by the sulphate process. A high solids slurry of TiO2 pigment produced by the sulfate process containing at least about 75% TiO2 pigment by weight may be obtained by first dry milling the TiO2 pigment then mixing the dry milled rutile TiO2 pigment with water, optionally, in the presence of an acrylic acid dispersant.
Dry milling TiO2 pigment produced by the sulphate process may be accomplished by using any number of devices and methods familiar to those skilled in the art. Preferably, the dry milling of the TiO2 pigment produced by the sulphate process is accomplished by pressure rolling the TiO2 in the manner described above respecting the pressure rolling of anatase TiO2 pigment.
The following examples set forth preferred embodiments of the invention. These embodiments are merely illustrative and are not intended to, and should not be construed to, limit the claimed invention in any way.